Ammonitinas or ammonites are the animals of group Ammonitida which leaved in the Mesozoic seas during Jurassic and Cretaceous periods. These are the cephalopoda mollusks, which name occurs from the name of ancient Egypt god Amon - spiral shell looks like the horns of the god depicted with the head of ram. Modern squids and octopuses are considered as Ammonitin kins.
Ammonites came from Litoceratides in the lower Jurassic (about 200 million years ago). This class was very various and included more than 600 genuses. Morphologically, the shells of the ammonites can be subdivided into the 3 types: smooth plate-spiral (genus Craspedodiscus see photo), hard textured plate-spiral (genus Speetoniceras see photo), heteromorphic (genus Audouliceras - see photo).
Ammonites disappeared 65 million years ago in the end of the Cretaceous period. Ammonites final extinction was at the same time as an extinction of dinosaurs and many other organisms of Mesozoic fauna.
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Hatched from the roe the ammonite had a very small shell 1 2 mm in diameter, whichs used as float. At the young stage of their life ammonites had a plankton mode of life. During the growth, the ammonite builds the new parts of shell in the front of it and moves his body in the shell tube to the wide end of the shell. In the back parts of the shell ammonite builds the septums subdivided the shell into the cameras (see photo of cut Speetoniceras). |
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The adult ammonites leaved in the different parts of the sea. Based on the morphology, the shells are subdivided into nekton, nekton-benthos and plankton species. Mollusk always occured out of the front part of shell only living camera. Other space subdivided by septums cameras (cleared from the ammonite body) at the back side of shell was filled by cameral liquid. The cameras in fragmakone were jointed by special tube siphon, in which the cameral liquid moved between the cameras. The fastening of the mollusk body to the walls of living camera was provided by special muscles retractors, which used for pull ammonite body into the shell and push it out of the shell. Sometimes, the prints of the muscles are found on the walls of living camera (see photo).
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The soft tissues of the ammonites were not fossilized. So we can only suppose the anatomy of the animal by the analogy of modern cephalopods, it means that the animals had a special funnel for jet movement in a water and not more then 10 tentacles (see photo). Many paleontologists think that ammonites swimed very slow, and they were accommodated to slow vertical movement in water. |
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Only shells and some other hard parts in the living cameras are saved (fossilized) from the ammonites. For example aptich (see photo) and fragments of paws beak of the ammonite. Morphology of the beak allows us to consider the ammonites as active predators or plankton-eating animals |
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But slow-movement ammonites exposed by the attacks of other predators sharks, fishes, reptiles. Now on the fossil shells we can find the evidence of the bites (see photo). Sometimes the traumas of the ammonites are so obviously and seemed to be non-compatible with an animal life, but ammonites made healthy again and continued to grow on a shell, which had after that an ugly form. (see photos). |
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After dying of the animal the ammonite shells felt down on the sea bottom and formed big burials. Then the shells were exposed by the fossilization. |
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During fossilization only small part of the shells saved completely, main part was deformed, split into fragments (see photos) or completely dissolved during the sedimentary rock forming processes. |
The taphonomic conditions which provide the saving of the ammonite shell are very unique and concerned with sublittoral sea sediments. The shells were plunged unto soft carbonate silt which cemented later into a rock that completely preserved the fossil. The classic region where these rocks outcrop is the Volga region of Russia, including Nigniy-Novgorod, Ulyanovsk and Saratov localities
In Ulyanovsk region Volga river flow cleaves the Middle Jurassic and Lower Cretaceous sediments at the length of more than 100 kilometers, which provide many outcrops with an abundance fossil material big sea reptiles, sharks and different fishes, and also cephalopods, gastropods, bivalvia molluscs.
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Main part of complete shells with good pearl layer is found in spherical shape limestone concretions (see photo), or covered by pyrite, lies into a viscous clay (see photos).
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The ammonites in concretions on Volga river banks |
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The material is collected from the bank terraces and outcrops on steep banks (see photo). |
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Then it is delivered by boat or feet to the workshop for restoration. |
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Using special tools ammonite is extracted from the concretion (see photo). Generally, more fragile central parts of the shells are not saved as a result of pressure during sedimentary rock diagenesis process or after destruction in transportation to the place of burial. The reconstruction of the ammonite includes the renewal of missed central part of the shell using a complete center from other ammonite. |
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